The Prime Minister told the Commons that the full HS2 project, which could cost as much as £106bn, although the sections beyond Birmingham to Manchester and Leeds will be reviewed in a bid to cut the bill.
Mr Johnson said he would appoint a minister to oversee HS2 delivery in a cabinet reshuffle confirmed for Thursday.
"If we start now, services could be running by the end of the decade,” he said.
"So today the Cabinet has given High Speed Rail the green signal. We are going to get this done.
"And to ensure that we do so without further blowouts on either costs or schedule we are today taking decisive action to restore discipline to the programme."
The Prime Minister admitted that the delayed project, which has seen its budget treble, had been beset by problems, but added: “Poor management to date has not detracted, in my view, from the fundamental value of the project."
SNP Westminster leader Ian Blackford called on the UK Government to throw its weight behind the an extension of the Borders Railway, which would link Edinburgh and Carlisle.
"If the Prime Minister is truly committed to rail connectivity across these islands, he should engage with the Scottish Government to improve rail links from Scotland to the major cities in the North of England such as Manchester, Newcastle and beyond,” Mr Blackford said.
“With the UK government seeking a review of the second phase to build the line in the north of England, Scotland must not be left last in line.
Mr Blackford hit out at the government’s “sheer mismanagement of the project, the spiralling costs attached to it, and the endless delays”.
And in a reference to a proposed bridge across the Irish Sea between Scotland and Northern Ireland, he added: “No number of Prime Ministerial vanity projects will ever heal the economic damage and the damage to connectivity which this Tory Brexit will inflict.
“The Prime Minister should provide the estimated £20 billion for this project to the Scottish Government and the Northern Ireland Executive so that they can spend these monies on their own priorities."